In order to meet the United States Golf Association (“USGA”) specifications, a golf ball must be spherical in shape and have equal aerodynamic properties and equal moments of inertia about any axis through its center. The ball must have a minimum diameter of 1.68 inches (4.267 cm), a maximum weight of 1.620 ounces (45.926 g), a maximum initial ball velocity of 255 feet per second, and travel a limited distance as measured on a standard USGA ball testing machine.
Until recent years, most commercially available golf balls have been two-piece or three-piece designs. Two-piece golf balls are comprised of a solid elastomeric core and a cover. Three-piece golf balls are comprised of a central core, which may be solid or liquid filled, surrounded by a polymeric material and a cover, or include a large diameter core and a two-layer cover. Three-piece golf balls also include wound balls, although this type of golf ball is no longer commercially available from major manufacturers. Other more recent designs, however, include four-piece and five-piece golf balls with most of these designs focusing on the layers near the cover layer.
Independent of configuration, most commercially available golf balls are made of nonmetallic materials such as elastomers, ionomer resins, polyurethanes, polyisoprenes, and nylons. Except for wound balls, these golf balls are made by injection molding and/or compression molding one layer around the core and/or around another layer. In order to obtain optimum playing characteristics, such as spin control and improved accuracy (i.e., fewer hooks and slices without sacrificing distances), golf ball designs and their materials of manufacture are becoming increasingly complicated.
The presence of a relatively incompressible metal core in a golf ball subjects the surrounding polymer layer to unusual conditions and stresses that are not encountered in golf balls having compressible polymer cores. When a golf club strikes a metal core golf ball, the polymer layer is compressed against the metal core, and since the core does not yield to any significant degree, the polymer tends to be displaced in a direction parallel to the surface of the core. An excessive amount of such displacement can break the bond between the core and the surrounding polymer layer, resulting in delamination. The polymer may also be fractured by the unusually large stresses put upon it when compressed between a face of a golf club and a metal core.
Depending on the design, hollow metal core golf balls may have shortcomings, including the aforementioned low durability, hard feel, and a small loss of distance compared to more typical molded balls discussed above. Accordingly, there is a need for golf balls that have a hollow metal core that do not exhibit some or all of the shortcomings of these golf balls appearing in the art. Furthermore, there is a need for materials that allow a golf ball designer to use high stiffness cores, while avoiding the shortcomings observed in such golf balls. In addition, there is, accordingly, a need for polymer compositions and/or design approaches that are especially adapted for use with metal core golf balls. Perhaps most importantly, needs should be met in a manner that allows an economical means of production.